<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div class="container">
        <h2 class="page-header">Promise封装AJAX操作</h2>
        <button class="btn btn-primary" id="btn">点击发送AJAX</button>
    </div>
</body>
</html>  
<script>
    //接口地址 https://api.apiopen.top/api/sentences
    //获取元素对象
    const btn = document.querySelector('#btn');
    btn.addEventListener('click', function () {
        const p = new Promise((resolve, reject) => {
            const xhr = new XMLHttpRequest();
            xhr.open('GET', 'https://api.apiopen.top/api/sentences');
            xhr.send();
            xhr.onreadystatechange = function () {
                if (xhr.readyState === 4) {
                    if (xhr.status >= 200 && xhr.status < 300) {
                        resolve(xhr.response);
                    } else {
                        reject(xhr.status);
                    }
                }
            }
        });
        p.then(value => {
            console.log(value);
        }, reason => {
            console.warn(reason);
        })
    });
</script>